RFR: 8343336: Add persistentScrollBars preference [v6]

[Michael Strauß]

> This PR adds another accessibility preference that is available on all supported desktop platforms:
> 
> 1. **Windows**: Settings -> Accessibility -> Visual Effects -> Always show scroll bars
> 2. **macOS**: System Settings -> Appearance -> Show scroll bars
> 3. **Ubuntu**: Settings -> Accessibility -> Seeing -> Always show scroll bars
> 
> When the OS setting is changed, all platforms report updates that can be observed in JavaFX.
> The manual test application `test/manual/events/PlatformPreferencesChangedTest` can be used to test the feature.

Michael Strauß has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into feature/accessibility-hide-scrollbars
 - check whether gtk-overlay-scrolling exists
 - Merge branch 'master' into feature/accessibility-hide-scrollbars
 - add platform keys to javadoc
 - Change reducedScrollBars to persistentScrollBars
 - Add reducedScrollBars preference

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1618/files
  - new: https://git.openjdk.org/jfx/pull/1618/files/1f194ce2..6d44e280

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1618&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1618&range=04-05

  Stats: 7644 lines in 194 files changed: 4087 ins; 2699 del; 858 mod
  Patch: https://git.openjdk.org/jfx/pull/1618.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1618/head:pull/1618

PR: https://git.openjdk.org/jfx/pull/1618